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Recent measurements of sea ice in the Arctic Ocean have found that it has melted to its lowest volume in recorded history.

Although the area of Arctic sea ice is greater than at the corresponding period in 2007 - the lowest coverage ever - experts are concerned that it may be much thinner.

Dr Walt Meier of US National Snow and Ice Data Center and colleagues say the overall ice volume in the Arctic Ocean is at least as low as 2007, and may even have dwindled more by as far as 10%.

Meier says this makes the ice more prone to melting than ever before.

Steady decline

The area of ocean that sea ice covers can vary dramatically from year to year, so scientists look at ice volume as a long-term indicator of the "health" of the ice.

Since satellite measurements began 30 years ago, it has been in steady decline.

"The long term trends in ice volume show a clear trend toward warming temperatures," says Meier. "The health of the ice is at the same state or worse than it was last year."

The latest drop in volume is likely due to a strong wind pattern last year that blew large amounts of thick, multi-year ice south past the east coast of Greenland, and out of the Arctic Ocean. During the following winter, thin new ice formed in its place.

But given the ever-warming waters and air temperatures, Dr Jay Zwally of NASA's Goddard Space Flight Center says the new ice has little chance of making a long term recovery.

"The reason volume is so important is new ice can't get thick enough in the winter to survive next summer's melting," he says. "It takes seven to eight years for sea ice to reach its equilibrium thickness of around four to five meters."

Last December, Zwally predicted that the Arctic Ocean could be seasonally ice free by the end of the summer of 2013, five years from now.

Shielding effect

It would be more than just a symbolic milestone of the warming Earth.

Pack ice reflects sunlight, which protects the water below from warming. As the ice dwindles so does its shielding effect. This means the less ice there is, the more quickly the ocean warms.

This factor, called the ice-albedo feedback, can exacerbate the effects of global warming.

The latest measurements are preliminary, though they seem to confirm Zwally's suspicions.

He says scientists will have a better handle on the situation later this year, once the data are in from the ICE Sat satellite, which is currently taking readings of ice thickness from orbit.